Firefox 4 was originally slated for release in late 2010.
Today Mike Beltzner of Mozilla has confirmed what many has been suspecting - Firefox 4 will be delayed. He announced that due to the delay in the release of a feature complete seventh beta, which is needed by the third part developers etc. to develop add-ons etc., the schedule has been pushed back.
According to the revised schedule, the seventh beta is expected in November 2010 and the release candidate in early 2011. The final release will follow soon after the release candidate.